Knowing Different Global Causes Could Aid Future Drought Warnings

Article by Jana Goldman, NOAA Office of Communications

October 13, 2009

Dust buried farms and equipment, killed livestock, and caused human death and misery during the height of the Dust Bowl years.

After analyzing historical records and climate model data for two major
U.S. droughts in the 1930s and 1950s, NOAA scientists found two very
different causes, shedding new light on our understanding of what triggers
drought.
Studies such as this one that expand our insights into drought are
essential for improving forecasts and can aid in the creation of an early
warning system to help communities take precautions and prepare.

"In the case of the severe 1950s drought of the Southern Plains states, it
appears global sea surface temperatures were the principal cause. However,
the 1930s 'Dust Bowl' drought over the central and northern Plains states
was not caused by ocean conditions, but rather the evidence points to
random changes in the atmosphere as the instigator in that event," said
Martin Hoerling, lead author and a meteorologist at NOAA's Earth System
Research Laboratory in Boulder, Colo.

The work, "Distinct Causes for Two Principal U.S. Droughts of the 20th
Century ," is currently available online and will be published in the October 16 edition of Geophysical Research
Letters. [Abstract]

Scientists studied two national events – one over the Southern Plains
during 1946-1956 and the other in the central and northern Plains during
1932-1939, commonly known as the Dust Bowl period. The authors note that
the two events are considered the most severe and prolonged droughts over
the Great Plains since 1895. In the United States, the Plains encompass
roughly the area west of the Mississippi River and east of the Rocky
Mountains.

It has been long held that droughts are influenced by sea surface
temperatures. But the NOAA scientists saw differences in the two droughts
they studied and delved deeper into the causes.

Our finding that the 1930s Dust Bowl drought was likely caused by a random
change in the atmosphere does conflict with earlier scientific studies,Ó
said Hoerling. "But in our analysis, using a more extensive set of model
experiments and diagnostic tests, the prior speculation that the Dust Bowl
drought had early warning indicators in the ocean temperatures could not be
supported."

However, the evidence surrounding the Southern Plains drought does point to
sea surface temperatures as the driver for that decade-long event that
began in 1946. The authors note that the Southern Plains region is more
sensitive to sea surface temperatures than the Northern Plains due to its
proximity to tropical Pacific Ocean-influenced weather patterns.

"Both the observations, as well as the simulations show strong and frequent
La Ni&ndildea;s during Southern Plains drought," said Hoerling. "Droughts are
common in that region during La Niña years."

La Niña is
characterized by unusually cold water temperatures in the
equatorial Pacific Ocean as opposed to El Niño,
which reflects unusually warm temperatures. These events can affect global weather patterns; in the
United States, La Niña often signals drier-than-normal conditions in the
southwest and central Plains regions, and wetter fall and winter seasons in
the Pacific Northwest.

The authors note that while an ocean observing system – a network of
instruments including stationary and free-floating buoys as well as
satellites – is vital to any drought early warning system, it may not
adequately warn of a drought caused by other factors, and may not provide
early warning for a drought over the northern Plains states, such as what
occurred in the 1930s.

Xiao-Wei Quan and Jon Eischeid from the Physical Sciences Division of
NOAA's Earth System Research Laboratory in Boulder, Colo., also are authors
on the paper.